Angle-measuring instruments of the kind referred to above equipped with optical-analog read-out means for the horizontal circle require an arrangement for positioning the circle so that when the instrument is set up, a predetermined read-out value can be set with respect to a predetermined direction in the topography. For this reason, the horizontal circle of these instruments is rotatable with respect to the stationary lower portion of the housing as well as with respect to the upper portion of the instrument with which the telescope is associated.
With instruments of lower and medium accuracy, the circle part is coupled via a repetition clamp with the upper part of the instrument and is moved along by the drive of the upper part. Such repetition clamps are also known as so-called Mahler clamps.
Instruments of medium, high and highest accuracy are provided with a separate circle drive for setting the desired direction indication with the upper and lower parts held stationary. To enable the circle part to be set rapidly with the needed accuracy, this drive is configured in part as a two-stage drive. With a coarse drive, the circle part can be adjusted over the entire angle range of 360.degree.; whereas, an additional fine drive is provided for finely setting the circle part in a limited angle range.
Such a drive for the circle part is, for example, described on page 107 of the text entitled "Instrumentenkunde der Vermessungstechnik", 5th Edition, by Dr. Fritz Deumlich and published by VEB Verlag fur Bauwesen, Berlin, 1972. In this arrangement, a first adjusting drive arrangement functions as a coarse drive and rotates the carrier of the circle part via a spur gear system; whereas, a high step-down lever gear system superimposes the movement of a second adjusting drive on the coarse drive. The second adjusting drive functions as a fine drive and acts on the carrier of the circle part by means of a slip coupling.
It is however difficult and requires additional means to configure both transmissions so that they do not interfere with each other in order to enable the circle part to be preset with satisfactory accuracy. Such means are not disclosed in the description provided by the above-identified reference.
German Pat. No. 12 16 554 discloses a device for automatically setting a shaft to a predetermined angular value. In this arrangement, a fine adjust knob rotates two plane-parallel plates in respective optical paths. The paths extend between respective sets of graduations of a graduated circle and respective photoelectric read-out devices. The read-out devices provide a signal to energize a motor coupled to the shaft of the coarse drive. The motor rotates the shaft an amount corresponding to the fine preset angle value.
In this arrangement, the precision of the shaft position setting is determined by the motor-driven coarse drive. Furthermore, the configuration of the apparatus is complicated.